1. Field of the Invention
The present invention relates to a developing apparatus for use in electrophotographic or xerographic equipments such as a copying machine, a printer or a facsimile system.
2. Description of the Related Art
FIG. 6 illustrates an example of a conventional developing apparatus which uses a dual component developer containing carrier and toner particles. The conventional developing apparatus 2 of this figure is comprised of a vessel (or casing) 30 which incorporates a developing roller 5 and contains a developer, a regulating plate 6 placed in parallel with the axis of rotation of the developing roller 5 at a predetermined distance from the circumferential surface of the developing roller 5, an agitating roller 10 which is similarly incorporated in the vessel 30 and is operative to agitate the developer contained in the vessel 30, a toner hopper 21 which is fixedly supported in the vessel 30 and stores toner particles therein, a supplying roller 22 which is provided in the toner hopper 21 and is operative to supply toner into the vessel 30, a cylindrical photo-conductive body (namely, photoconductor) 1 placed in such a manner as to have the axis of rotation thereof parallel to the axis of rotation of the developing roller 5, and a means or device (not shown) for forming an electrostatic latent image along the surface of the photo-conductive body 1.
The developing roller 5 facing the photo-conductive body 1, the axis of rotation of which is parallel with that of the roller 5, is constructed by fixedly placing a cylindrical multipole permanent magnet 4 in a rotatable developing sleeve 3. A plurality of magnetic poles S, N, S, N, . . . are provided on a peripheral portion of the multipole permanent magnet 4. Further, a developing magnetic pole portion (namely, double magnetic poles having the same polarity) 8 is provided in another portion of the multipole permanent magnet 4 to face the photo-conductive body 1.
In the case of this developing apparatus, the dual component developer 7 containing carrier and toner particles is attracted by the force of attraction of the multipole permanent magnet 4 and is then held on the periphery of the sleeve 3. This developer is carried by the rotation in the direction of an arrow A (namely, the counterclockwise direction as viewed in this figure) of the sleeve 3. When this developer passes through the gap between the regulating plate 6 and the sleeve 3, superfluous developer, namely, an outer layer of the developer outside the inner layer thereof, whose thickness is equal to the width of the gap, held on the periphery of the sleeve 3, is scraped away therefrom. Thus, developer of uniform thickness, which is deposited on the periphery of the sleeve 3, is carried to the developing magnetic pole portion 8. In this developing magnetic pole portion 8, the developer 7 forms a magnetic brush along lines of magnetic force and further, a toner image is formed on the surface of the photo-conductive body 1 by bringing the developer 7 into contact with the electrostatic latent image formed on the photo-conductive body 1 which rotates in the direction of an arrow B (namely, clockwise, as viewed in this figure). Furthermore, the toner is consumed in the development. Therefore, the vessel 30 is replenished with toner particles through the supplying roller 22 from the toner hopper 21, if necessary.
Moreover, in order to increase the developability in the developing magnetic pole portion 8, there has been proposed a method by which the developing magnetic pole portion 8 is constituted by two adjoining magnetic poles, namely, double magnetic poles having the same polarity. In the space between such double magnetic poles, the constraint on the developer 7 due to magnetic force (namely, magnetic field strength) is removed owing to the presence of a repulsive magnetic field. Thus the developer 7 comes to easily move to the photo-conductive body. Consequently, the developability can be increased even in the case where there is soft contact between the developer and the photo-conductive body. Such developing methods with double magnetic poles are disclosed in, for example, the Japanese Patent Public Disclosure Official Gazettes Nos. 55-101969/1980, 3-291680/1991 and 4-338781/1992. Moreover, there have been proposed other methods such as a method using AC bias to increase image density (see the Japanese Patent Public Disclosure Official Gazettes Nos. 61-198170/1986, 60-168177/1985 and 3-109582/1991).
The aforementioned developing apparatus, however, has the following propensity. Namely, when using carrier particles having small magnetizing force or those of small diameters in order to obtain a sober contact between the developer 7 and the photo-conductive body 1 and secure uniform print quality, the force of constraint of the developer due to the developing magnetic pole is weakened so that carrier particles come to deposit on the photo-conductive body 1 and the print quality is deteriorated. Besides, in the aforementioned Official Gazettes concerning the developing method with double magnetic poles, for instance, the Japanese Patent Public Disclosure Official Gazette No. 55-101969/1980, there is illustrated an example in which two peaks of magnetic field occur in the developing magnetic pole portion. This Official Gazette, however, makes no mention of the relation between the developing point at which the distance from the developing roller to the photo-conductive body becomes minimum and the distribution of magnetic field having two peaks thereof.